Heretofore compensation for expansion and contraction due to temperature changes within plastic piping systems has been accomplished by using expansion joints or offset loops. While these techniques are acceptable for use in single wall piping systems, with the advent of "double wall piping" systems (i.e., double containment or dual containment piping), compensation for material expansion and contraction has required other solutions. Ziu U.S. Pat. No. 4,786,088, describes the use of restraint couplings for controlling expansion and contraction in double wall piping systems of similar materials. This system is unsuitable for dissimilar materials due to a lack of acceptable joining techniques. Other systems by Plexco (Chevron), George Fisher, and Asahi (as well as Ziu) recommend the use of expansion loops which are both costly and inadequately address the problem of expansion of dissimilar materials.
Standard bellows or piston type expansion joints are not suitable for double wall piping systems due to space limitations of the containment or outer pipe.